Subject: Re: [PATCH v7 net-next 5/5] net: ethernet: ti: cpsw: add XDP support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 5 Jul 2019 13:13:54 +0200

On Fri,  5 Jul 2019 02:14:06 +0300
Ivan Khoronzhuk <[email-masked]> wrote:

> +static int cpsw_xdp_tx_frame(struct cpsw_priv *priv, struct xdp_frame *xdpf,
> +			     struct page *page)
> +{
> +	struct cpsw_common *cpsw = priv->cpsw;
> +	struct cpsw_meta_xdp *xmeta;
> +	struct cpdma_chan *txch;
> +	dma_addr_t dma;
> +	int ret, port;
> +
> +	xmeta = (void *)xdpf + CPSW_XMETA_OFFSET;
> +	xmeta->ndev = priv->ndev;
> +	xmeta->ch = 0;
> +	txch = cpsw->txv[0].ch;
> +
> +	port = priv->emac_port + cpsw->data.dual_emac;
> +	if (page) {
> +		dma = page_pool_get_dma_addr(page);
> +		dma += xdpf->data - (void *)xdpf;

This code is only okay because this only happens for XDP_TX, where you
know this head-room calculation will be true.  The "correct"
calculation of the head-room would be:

  dma += xdpf->headroom + sizeof(struct xdp_frame);

The reason behind not using xdpf pointer itself as "data_hard_start",
is to allow struct xdp_frame to be located in another memory area.
This will be useful for e.g. AF_XDP transmit, or other zero-copy
transmit to go through ndo_xdp_xmit() (as we don't want userspace to
be-able to e.g. "race" change xdpf->len during transmit/DMA-completion).

> +		ret = cpdma_chan_submit_mapped(txch, cpsw_xdpf_to_handle(xdpf),
> +					       dma, xdpf->len, port);
> +	} else {
> +		if (sizeof(*xmeta) > xdpf->headroom) {
> +			xdp_return_frame_rx_napi(xdpf);
> +			return -EINVAL;
> +		}
> +
> +		ret = cpdma_chan_submit(txch, cpsw_xdpf_to_handle(xdpf),
> +					xdpf->data, xdpf->len, port);
> +	}

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer